The 3GPP (3rd Generation Partnership Project) is currently considering introduction of an E-UTRAN (Evolved Universal Terrestrial Radio Access Network) system which is an extended model of a W-CDMA system. The examination is made based on an implementation that OFDMA (Orthogonal Frequency Division Multiple Access) is applied to a downlink air interface in the E-UTRAN system.
DS-CDMA (Direct Spreading-Code Division Multiple Access) has been used in the existing W-CDMA system. In the DS-CDMA, signals are transmitted while being subjected to code diffusion and, on the reception side, a desired signal is subjected to back diffusion using the diffusion code to increase a received power density of the desired signal required per symbol, thereby enhancing SIR (Signal-to-Interface Ratio) at the receiving end.
Further, in the W-CDMA system, a scrambling code unique to each cell is multiplied with transmission data. The mobile station knows the scrambling code of the cell to which the own station is connected and receives the transmission data using the scrambling code unique to each cell to which the mobile station is connected. Signals from adjacent cells are still scrambled, and interference signals are randomized, so that even when the data is transmitted from the adjacent cells using the same frequency band and same diffusion code, inter-cell interference can be effectively reduced by the diffusion. Therefore, so-called one-cell reuse in which the same frequency is used in the entire system can easily be achieved.
On the other hand, in a system using the OFDMA, a frequency band used for data transmission is divided into a plurality of orthogonal frequency bands (sub-carriers), and respective sub-carriers are modulated and multiplexed in data transmission, thereby realizing large-capacity transmission. Therefore, as shown in FIG. 1, sub-carriers having satisfactory reception quality and those having poor reception quality exist together in each sub-carrier group due to influence of frequency selective phasing. Thus, in the OFDMA, it is important to stabilize communication quality using a combination of error correction and interleave techniques. Further, since the principle of the OFDMA is that interference from other signals is eliminated using orthogonality of sub-carrier, when data is transmitted using the same sub-carrier from the adjacent cells, large interference is caused between the cells to significantly degrade communication quality. Thus, in a wireless LAN system and the like using the OFDMA, previous setting is required such that the same channel (frequency block composed of a predetermined number of sub-carriers) is not used by adjacent access points. However, when such a multiple-cell repeat is used, a frequency band that can be used by each cell becomes narrower than the entire frequency band that can be used by the entire system, decreasing the frequency use efficiency of the entire system.
Thus, it is proposed that a system shown in FIG. 2 or FIG. 3 is used in the E-UTRAN system to increase the frequency use efficiency of the entire system.
In the system shown in FIG. 2, a state close to one-cell reuse is achieved at the center of each cell while multiple-cell reuse is used at the boundary of each cell so as to restrict a use of a part of the frequency band (refer to, e.g., Non-patent Document 1). That is, at the portion near the cell boundary, a frequency block (FB) other than FBs used by the adjacent cells is used.
Similarly, in the system shown in FIG. 3, all frequency blocks (FBs) are used at the center of each cell while an FB other than FBs used by the adjacent cells is used at the boundary of each cell (refer to, e.g., Non-patent Document 2). In FIGS. 2 and 3, one obtained by bundling a predetermined number of sub-carriers as shown in FIG. 4 is represented as a frequency block (FB), and it is assumed that a plurality of FBs (e.g., FB0 to FB12) can be used in the entire system.
It is now being considered that a mobile station in the E-UTRAN system measures the reception SIR of a pilot signal for each FB and transmits a CQI (Channel Quality Indicator) representing the channel quality of the FBs to each connection base station. Since the CQI measurement value includes interference caused by adjacent base stations, the CQI of the FBs used by adjacent base stations are deteriorated. Thus, it is believed that by designing a configuration that the FBs exhibiting a low CQI value is not transmitted, interfere between adjacent base stations can be reduced to some degree.    [Non-Patent Document 1] 3GPP TSG RAN WG1 LTE Ad Hoc meeting, R1-050594 Multi-cell Simulation Results for Interference Co-ordination in new OFDM DL, Alcatel    [Non-Patent Document 2] 3GPP TSG RAN WG1 LTE Ad Hoc meeting, R1-050599 Multi-cell Interference mitigation—Considerations and Results on Frequency Reuse, Siemens